Current refinery feedstocks more and more frequently include such products as straight-run or vacuum residues of conventional oils, crude or topped oils or even shale or bituminous sand oils.
These various feedstocks are characterized by a very high asphaltene and "resin" content. These products, which also contain other heteroatoms such as sulfur and nitrogen, with metal complexes of vanadium or nickel, make very difficult the conventional refining of such feedstocks.
In catalytic processes, hydrotreatment performance quickly decreases as a result of nickel and vanadium sulfide deposits on the catalysts, which poison them. In catalytic cracking using zeolite cataysts, the high Conradson carbon content of said feedstocks results in coke deposits requiring an increased temperature for catalyst regeneration. Moreover, the overly high nickel and vanadium contents of the feedstocks have some negative effects (substantial gas formation, modification of the catalysts resulting in a loss of their activity). In hydrocracking, the feedstock must not contain more than a very small proportion of asphaltenes in order to avoid quick poisoning of the catalyst active sites.
In the visbreaking thermal process, the severity of the operation conditions depends on the asphaltene content of the feedstock. The coagulation of the partially cracked asphaltene molecules results in instability of effluents, which tend to settle during storage and to clog filters.
All these disadvantages have induced the refiners to separate asphaltene and resin compounds from oily fraction containing them. This separation is achieved by the so-called solvent deasphalting technique which consists of breaking the prevailing balance between the asphaltenes and the maltenic medium by adding to the feedstock a solvent which decreases the interfacial tension and the viscosity of the medium.
For this purpose, light paraffins or olefins containing 3 to 7 carbon atoms are mostly used, either alone or as an mixtures. These products act as antisolvent for asphaltenes, and for any resins any.
For a given feedstock, the yield to deasphalted oil and its quality depend on the type of solvent, on the volume ratio of the solvent to the feedstock and on the temperature and pressure of the deasphalting operation.
The composition and the characteristics of the phase which precipitates during the deasphalting operation, called the "asphalt phase", may thus vary within a very wide range. This asphalt phase may be roughly divided into two categories of compounds, one, called "asphaltenes", defined as all the products which precipitate by means of excess n-heptane, according to Standard AFNOR T60-115, the other, called "resins", defined as all the products insoluble in propane but soluble in heptane. It is well-known that asphaltenes contain the major part present in the metals (nickel and vanadium) of heavy oils.
On the other hand, in an economical deasphalting process, the asphalt fraction must be as low as possible for a given quality of deasphalted oil. It is known that, everything else being unchanged, the yield of asphalt decreases with an increasing molecular weight of the deasphalting solvent. Presently, a solvent such as the so-called C.sub.5 cut, essentially consisting of pentane and isopentane, is more and more frequently used. For a given feedstock, the use of propane instead of pentane results in an increased yield of deasphalted oil, since the latter will contain a part of the "resins". A good compromise can nevertheless be obtained between quality and quantity of the obtained deasphalted oil.
With respect to the asphalt phase, the tendency to use deasphalting solvents of higher molecular weight results in a decrease of the yield by weight, corresponding to a lower resin content. As far as quality is concerned, the asphalts precipitated according to said method generally have softening points (measured according to the BaLL and Ring method, Standard AFNOR T 66-008 higher than 100.degree.-120.degree. C., and which may be as high as 180.degree. C.-200.degree. C.
Such asphalts are difficult to use. Their softening point is too high for economical use as a bitumen covering for roads. Their combustion as fluxed fuels, without special adaptation of the conventional combustion units, produces an amount of unburnt particles incompatible with the legal requirements; moreover, their relatively high softening point requires dilution with a substantial amount of fluxing agent.
As solid fuel, their softening point, mostly of about 120.degree.-160.degree. C., is too low for an easy use in units of the fluid bed type. Thus, the division of said phase into two fractions, one of which has a softening temperature lower than that of the initial asphalt, associated with a lower metal content, and the other a very high softening temperature, e.g. higher than 250.degree. C., is economically advantageous.
By the process according to the present invention, solid asphalts, as particles suspended in an aqueous medium, are divided into two fractions by addition of a solvent immiscible with water, so as to obtain a first asphalt fraction in the solvent and a second asphalt fraction suspended into the aqueous medium, having such a viscosity that said suspension can be easily conveyed and pumped.
Many documents of the prior art disclose the fractionation of heavy oils into three fractions: asphaltenes, "resins" and deasphalted oil, by the action of at least one deasphalting solvent, these separations being performed in several successive steps.
This technique is disclosed, for example, in U.S. Pat. No. 2,940,920, where a single deasphalting solvent is used; briefly stated, it consists of subjecting the feedstock, in a first step, to the action of a light paraffinic or olefinic solvent in excess, under such conditions as to separate in said step, by settling, a lower asphalt phase and an upper oily phase. In a second step, the oily phase obtained in the first step is brought to higher temperature and pressure, producing the separation between the lower phase formed of resins and an upper phase comprising the deasphalting solvent and the residual oil. These two constituents are separated in a third step under supercritical conditions adapted to separate the solvent from the deasphalted oil.
This process, usually called "Rose Process" has been the object of many patents disclosing different operating conditions, or the use of several solvents as specified, for example, in U.S. Pat. Nos. 3,830,732 and 4,125,459; many papers have also disclosed said technique, one of the most recent being that of S. R. NELSON and R. G. ROODMAN in CHEMICAL ENGINEERING PROGRESS of May 1985, p. 63.
This process suffers from two major disadvantages; firstly, it requires, from an economical point of view, high investment costs since the fractionation of the feedstocks into their three main constituents is performed at high temperature and pressure conditions, secondly, it is not adapted to produce substantially asphalts having a softening temperature higher than about 200.degree. C., the obtained products having such a viscosity that they cannot be pumped, even when heated to temperatures of about 300.degree. C.